What is the simplest way to increase volume of mesh?
In Edit Mode, you can use the Shrink/Fatten tool. &#9095; AltS
It is limited to the selection.





You can also add a displacement modifier and change the strength value.
For more control assign a vertex group to it. Create a the group and go into weight paint mode &#9096; Ctrl&#8633; Tab to paint the weights.


It seems the gif progress bar got messed up. Sorry.